Systems and methods of securing immunity to air CO2 in alkaline fuel cells

ABSTRACT

An air CO 2  filtration assembly or system is provided that includes CO 2  filters or traps designed and configured with a limited, but high capacity, volume to maximize filtration/absorption of CO 2  from an air stream supplied to an alkaline fuel cell to thereby minimize the CO 2  level in the air stream fed into the fuel cell cathode. The CO 2  filters or traps include at least one thermally regenerative CO 2  chemical filter or trap arranged in a tandem configuration with a strongly bonding CO 2  chemical filter or trap. The combination of the two types of filters or traps sequentially filter/absorb CO 2  from the air stream and reduce the level of CO 2  in the air stream fed into the cathode. The air CO 2  filtration assembly or system may be used in conjunction with electrochemical purging of the alkaline fuel cell that enables removal of CO 2  from the fuel cell by anodic decomposition of accumulated carbonate ions in the fuel cell anode and release of CO 2  through the anode exhaust stream.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/862,746, filed Aug. 24, 2010, which claims priority to U.S.Provisional Application Ser. No. 61/236,282, filed Aug. 24, 2009.Priority is claimed to both U.S. application Ser. No. 12/862,746 andU.S. Provisional Application Ser. No. 61/236,282 and the entireties ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The inventions provide air CO₂ filtration/absorption assemblies andsystems for use with an alkaline fuel cell to reduce levels of CO₂ inair streams supplied to the fuel cell cathode. The inventions alsoprovide methods of electrochemical purging through application of apurging current to an alkaline fuel cell to achieve electrochemical CO₂removal from the fuel cell. The inventions are for airfiltration/absorption in alkaline fuel cells that include alkalineaqueous electrolyte or Off ion-conducting polymeric membranes withoutliquid electrolyte.

BACKGROUND

Alkaline membrane fuel cells (AMFCs) have important advantages overother low temperature fuel cells, including the ability to operate withnon-precious metal catalysts and without added liquid electrolyte.However, an important challenge to the implementation of this fuel celltechnology is the performance loss incurred when CO₂ enters the cell.When the AMFC operates on hydrogen fuel, the CO₂ in the cathode air feedis a specific source of concern, as the air feed contains around 400 ppmCO₂. This “air CO₂” will enter the cell continuously through the cellcathode, as long as such untreated air supplies the cathode. Under suchconditions of continuous inflow of CO₂ at a partial pressure of about10⁻⁴P_(air) into the cell cathode, and from the cathode into the cell,significant AMFC voltage losses have been recorded. The cell voltage atconstant current density of about 0.2 A/cm²-0.4 A/cm² is found to belower by 0.1V-0.3V (in contrast to the same cell operating with aCO₂-free cathode air feed), and has been shown to amount to a loweringof the energy conversion efficiency by 20%-60%.

One reason for this fall in AMFC performance is understood to be anacid-base process. CO₂ entering the cell recombines with the basicfunction of the polymer electrolyte to replace the OH⁻ ion-conductingfunction with a HCO₃ ⁻ (bicarbonate) ion conducting function accordingto:CO₂+(R₄N⁺OH⁻)=(R₄N⁺HCO₃ ⁻)  (1)

R₄N⁺ is a tetra-alkyl ammonium ion, the typical immobilized cationicgroup in an alkaline ionomer. After entering the cell cathode in gaseousform, CO₂ can migrate through the thickness dimension of the cell inwater-dissolved form, and can propagate the “carbonation process” shownby equation (1) throughout the membrane and the anode of the cell.

Another mode of propagation of the carbonation process through thethickness dimension of the cell is an anion-replacement process. In thiscase, a bicarbonate anion migrates through the ionomer under current,displacing an OH⁻ anion according to:HCO₃ ⁻+(R₄N⁺OH⁻)=(R₄N⁺HCO₃ ⁻)+OH⁻  (2)

This occurs while OH⁻ ions in the AMFC migrate towards the cell anodeand the anode process consumes OH— ions according to:H₂+2OH⁻=2H₂O+2e  (3)where the HCO3⁻ anion is not reactive at the anode under ordinary AMFCoperation conditions.

Consequently, the ion-replacement process (2), occurring while the anodeconsumers OH⁻ ions, will end up in lasting carbonation of a largefraction of the anionic sites.

Replacement of the OH⁻ anion by HCO3⁻ may cause significant AMFC lossesfor two reasons. First, the mobility of the bicarbonate ion is about 4times smaller than that of the OH⁻ ion, causing a drop of conductivityin both the cell membrane and the inner-electrode ionomer components. Asecond reason is the carbonation of OH⁻ ions within the anode. With theOH⁻ ion serving as a reactant in the anode process, lowering itsavailability for the anode process, as shown in equation (3), results ina significant increase of the anode over-potential.

Electrolyte carbonation is well documented as a significant challenge inalkaline fuel cells (AFCs) based on liquid alkaline electrolytes, e.g.,aqueous KOH. The nature of the problem and the solutions required,however, are different in AFCs and in AMFCs. In the case of the AFC, theultimate result of electrolyte carbonation is the formation of solidcarbonate in the liquid electrolyte that needs to be removedcontinuously. This is typically accomplished with continuous electrolyterecirculation and solid/liquid separation. In the AMFC, no solidcarbonate can be formed, which eliminates the need for liquidrecirculation and solid carbonate removal. However, the reaction of airCO₂ with the liquid alkaline electrolyte to form solid carbonateprovides a CO₂ sequestration function within the cell. Because the AMFCdoes not have such in-the-cell CO₂ sequestration function, the ionomermaterial in the AMFC becomes highly vulnerable to air CO₂ and thecarbonation processes shown in equations (1) and (2) readily convert theionomer on entry of untreated air from an OH⁻ ion form to a carbonateion form. Therefore, blocking entry of CO₂ and use of remediation toolswith an alkaline fuel cell that suffers some degree of carbonation mustconsequently be effective in securing the cell's immunity to air CO₂.

Other than electrolyte recirculation, the traditional approach tominimize the effects of CO2 in alkaline fuel cells has been the upstreamuse of air scrubbers containing aqueous alkaline solutions or solid CO₂absorbers consisting of granules of alkali and/or alkaline earthhydroxides, such as disclosed in U.S. Pat. No. 3,909,206. When passingthrough such scrubber or absorber filters, the CO₂ component in the airfeed stream reacts with the OH⁻ ions in such CO₂ trap to form carbonatesand thereby to reduce the concentration of CO₂ in the air entering thecell. This mode of CO₂ filtration occurs upstream from the cell cathodeand requires periodic replacement of the filter or of the activematerial in the filter. The frequency of such manual replacements cannotbe too great in most fuel cell applications because of the need tominimize fuel cell maintenance. One possible way to lower the frequencyof filter replacements is to use filters having a larger volume, i.e.,larger CO₂ absorption capacity. However, the permissible size of thefilter will be limited by the overall system volume constraints.

Thus, an effective CO₂ filter or trap having a combination of a limited,but high capacity, volume and a capacity to maximize a reduction of CO₂levels in an air stream by passing the air stream through such a filteror trap is desirable to minimize CO₂ levels in the air stream and withinan alkaline fuel cell.

SUMMARY

Applicants have identified that the demands of maximizing reduction ofthe CO₂ level in an air stream supply to an alkaline fuel cell aresignificantly more severe in alkaline membrane fuel cells (AMFCs) thanin alkaline fuel cells (AFCs). In an AFC, entry of scrubbed air havingthe CO₂ level remaining as high as 50 ppm may not cause strong fuel cellvoltage losses, particularly when the liquid electrolyte in the AFC isrecirculated. However, in the case of the AMFC, the CO₂ level must dropwell below 10 ppm to ensure near zero voltage loss. A single absorber,filter, trap, or other CO₂ filtration/absorption unit cannot achieve atits outlet such a low CO2 level in the air stream and have reasonabledimensions, when the air supply to the fuel cell is ordinary air havingabout 400 ppm CO₂. One reason is that the principles of filter designdirected to limiting filter dimensions and achieving a high gas flowrate are contrary to those principles that aim at perfect CO₂sequestration.

The inventions disclosed herein are directed to reducing the CO₂ levelin the air stream entering the cathode of an alkaline fuel cell to helpto secure a targeted efficiency level of the fuel cell and to achieveimmunity to CO₂ and its effects within the fuel cell. The inventions maybe used with an alkaline fuel cell including an alkaline aqueouselectrolyte (“AFC”) or an OH⁻ ion-conducting polymeric membrane withoutliquid electrolyte (“AMFC”). One object of the inventions includesproviding filter assemblies and methods designed and configured tosimultaneously minimize the volume size of the CO₂ filter(s) or trap(s)and to achieve a high throughput of the CO₂ filter(s) or trap(s), whileenabling the filter(s) or trap(s) to reduce the level of CO₂ level in anair stream supply to the fuel cell by a predetermined amount, e.g., by afactor of 10 or from about 400 ppm of CO₂ in ordinary (non-filtered) airto well under about 10 ppm CO₂. In addition, such methods may be used tohelp to achieve an “electrochemical purge” via the application of a highcurrent perturbation across the fuel cell, e.g., for a predeterminedtime, to purge through the anode exhaust stream any CO₂ penetrating thefuel cell.

In one aspect, the invention provides a two-filter CO₂ filtrationassembly including a combination of two types of CO₂ filters or trapsthat are operatively coupled with the fuel cell and are arranged in atandem configuration relative to one another. The two-filter assembly isupstream from the cathode of the fuel cell to reduce the level of CO₂ inthe air stream supplied to the cathode. More specifically, the filtersor traps are designed and configured to capture or absorb CO₂ in the airstream, as the air stream passes through the filters or traps, to reducethe CO₂ level in the air stream before it enters into the cathode. Thetwo-filter assembly thereby helps the alkaline fuel cell achieveimmunity to air CO₂ and, therefore, its targeted efficiency levels,through the assembly's absorption of CO₂ in the air stream supply to thecathode.

The two types of filters or traps of the two-filter assembly accordingto the invention may include a first thermally regenerative chemical CO₂filter or trap arranged in tandem with a second strongly bonding CO₂chemical filter or trap. The first thermally regenerative filter or trapis designed and configured for thermal regeneration upon CO₂ saturationwithout requiring disassembly of the filter or trap, as described below.As mentioned, the two types of filters are disposed upstream from aninlet to the cathode, and the second strongly bonding CO₂ filter or trapis disposed between the first filter and the cathode inlet. Thearrangement of the two-filter assembly with an alkaline fuel cellenables the first filter to receive the inlet air stream to be suppliedto the cathode and to reduce the level of CO₂ in the air stream as theair stream passes through the first filter. The arrangement also enablesthe second filter, disposed in tandem with and downstream from the firstfilter, to receive the filtered air stream exiting from the first filterto further reduce the level of CO₂ in the air stream as the air streampasses through second filter before the air stream is ultimatelysupplied to the cathode inlet. An air pump is included between the firstand the second filters to induce flow of an air stream through the twofilters or traps and into the cathode inlet.

The first thermally regenerative filter is designed and configured toreduce the level of CO₂ in the inlet air stream by a predeterminedamount, e.g., by a factor of 10. The second strongly bonding filter isdesigned and configured to reduce the level of CO₂ in the air streamfiltered by and exiting from the first filter by a second predeterminedamount, e.g., by a factor of 10. The air stream supplied to the cathodeinlet is thereby sequentially filtered by the first and second filters,such that, the level of CO₂ in the air stream is supplied to the cathodeinlet is significantly reduced, e.g., by a factor of 100, in oneconfiguration of the two-filter assembly.

For instance, in one configuration of the two-filter assembly accordingto the invention, the first thermally regenerative filter may beconfigured and designed to reduce the level of CO₂ in ordinary air by afactor of 10, or from about 400 ppm to about 40 ppm, and the secondstrongly bonding filter may be configured and designed to further reducethe level of CO2 in the air stream filtered by the first filter by afactor of 10, or from about 40 ppm to under 5 ppm, and preferably equalto or near 1 ppm. The two-filter assembly according to the invention maysignificantly reduce the level of CO₂ in the air stream supplied to thecathode inlet where ordinary air is used as the air supply to the fuelcell.

In another aspect, the invention provides a method of purging analkaline fuel cell electrochemically for CO₂ removal from the fuel cellanode through anodic decomposition. The method includes applying acurrent to the alkaline fuel cell suitable to help to forceparticipation of the accumulating carbonate ions in the anode as areactant in the anode process, thereby freeing CO₂ for removal from theanode through the anode exhaust stream. The magnitude of the current issufficiently high and just short of any magnitude that would cause anonset of fuel cell reversal in the stack. The method of electrochemicalpurging may be applied temporarily and periodically to the fuel cell. Inaddition, the method of electrochemical purging may be applied to analkaline fuel cell in response to a decrease in the fuel cell'sperformance over a given period of time, such as an operation time.According to the method of the invention, the application of theelectrochemical purging current may be for a predetermined duration,e.g., about 1 second to about 30 seconds. The OH⁻ ions are replaced bycarbamate ions as reactants in the anode process and are therebyconsumed electrochemically. The anode process releases CO₂ as aby-product and the anode exhaust stream releases CO2 from the fuel cell.The method according to the invention may be used advantageously withthe two-filter assembly described above, or with the CO₂ filtrationsystem, described below.

In a further aspect, the invention provides a CO₂ filtration system foruse with an alkaline fuel cell including the combination of the twotypes of CO₂ filters or traps, as described above, and further includinga second thermally regenerative CO₂ chemical filter or trap, similar tothe first thermally regenerative CO₂ chemical filter or trap. The firstand second thermally regenerative filters or traps are arranged inparallel and disposed upstream from the inlet to the cathode. Inaddition, each of the first and the second thermally regenerativefilters or traps is arranged in a tandem configuration relative to thestrongly bonding CO₂ chemical filter or trap. The first and secondthermally regenerative CO₂ filters or traps may be thermally rejuvenatedwithout their disassembly, as described above. The first or the secondthermally regenerative CO₂ filter, along with the strongly bondingfilter, filter the air stream as the air stream passes through eitherthermally regenerative filter and the strongly bonding filter, asdescribed above, to provide the air stream with a significantly reducedlevel of CO₂ to the cathode inlet. Each thermally regenerative filter isarranged in tandem with the strongly bonding CO₂ filter, and each ofthermally regenerative filter may be engaged in active CO₂ absorption,while the other thermally regenerative filter undergoes thermalrejuvenation. In this manner, one of the thermally regenerative filtersmay always be in service to filter the incoming air stream while theother thermally regenerative filter is being regenerated.

Thermal rejuvenation of the thermally regenerative filters isaccomplished by passing a warm or hot air stream through the thermallyregenerative filters to help to release absorbed CO₂ that builds up inthe filters during active operation. Thermal rejuvenation of thethermally regenerative filters can occur in-line, e.g., during operationof the fuel cell, whereby a warm or hot air stream passes through thefilter undergoing regeneration. Such warm or hot air stream may includethe cathode exhaust stream from the fuel cell that is redirected toeither of the first or second filter undergoing thermal rejuvenation.The CO₂ filtration system includes a subsystem of airflow lines andvalves that help to enable redirection of the cathode exhaust stream toeither the first or the second thermally regenerative filter, dependingon which of the first and second filters is designated for and/orundergoing thermal regeneration. The subsystem of airflow lines andvalves also helps to facilitate airflow of the inlet air stream toeither the first or the second thermally regenerative filter, dependingon which of the first and second filters is actively filtering, as wellas to direct air flow downstream from the filters to the stronglybonding CO₂ filter, and subsequently to the cathode inlet.

In some applications, with given filter properties and electrochemicalpurge conditions, complete filtration can be achieved by eliminatingeither the thermally regenerated filter or the strongly binding filterfrom the two-filter assembly or the CO₂ filtration system describedabove. The overall set of tools for elimination of CO2 effects wouldthen include a combination of the thermally regenerative filter and theelectrochemical purge, or the strongly bonding filter and theelectrochemical purge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an air CO₂ filtration assembly for analkaline fuel cell comprising two different types of CO₂ filters ortraps disposed in a tandem arrangement according to one aspect of theinvention;

FIG. 2 is a flow diagram illustrating a method of achieving CO₂ immunityin an alkaline fuel cell including CO₂ filtration of an air stream to acathode inlet and electrochemical perturbation of the fuel cell foranodic de-carbonation and release of CO₂ from the fuel cell through ananode exhaust stream;

FIG. 3 is a chart that illustrates a decrease in alkaline fuel cellvoltage, at constant current to the load, when a cathode air supplyprovided at “ultra zero” levels of any components other than oxygen andnitrogen is replaced with ambient air;

FIG. 4 is a schematic diagram of a CO₂ filtration system for an alkalinefuel cell comprising two thermally regenerative CO₂ filters or trapsarranged for thermal rejuvenation according to another aspect of theinvention; and

FIG. 5 is a flow diagram illustrating a method of determining minimumoverall dimensions of a design of the CO₂ filtration assembly or systemaccording to the invention to ensure reduction of CO₂ level in an airstream to cathode inlet.

DETAILED DESCRIPTION

The inventions provide assemblies and methods to effectively achievesubstantial alkaline fuel cell immunity to air CO₂ based on variouscombinations of features provided by: (a) chemical CO₂ filtrationthrough at least one high capacity/high throughput chemical CO₂ filteror trap that can be regenerated thermally without disassembly of thefilter or trap; (b) chemical CO₂ filtration through at least onedisposable, strongly bonding CO₂ filter or trap; and/or (c)electrochemical perturbation that helps to achieve anodic de-carbonationand release of CO₂ through the anode exhaust stream of the fuel cell.

Referring to FIG. 1, in one aspect, the invention provides a CO₂filtration assembly 50 for an alkaline fuel cell including a combinationof two types of filters or traps 12 and 14 arranged in a series or in atandem configuration relative to one another. The tandem configurationof two filters 12 and 14 is positioned downstream from an inlet 30 foran ordinary air supply to the assembly 50 and upstream from an inlet 32to the cathode of the fuel cell. The second or latter filter or trap 14of the two filters is positioned between the first filter or trap 12 andthe cathode inlet 32. An air pump 16 is disposed between the two filters12 and 14 to induce flow of an air stream through the two filters 12 and14 and into the cathode inlet. The two-filter assembly 50 shown in FIG.1 may be incorporated with an alkaline fuel cell that employs analkaline aqueous electrolyte or an OH⁻ ion conducting polymeric membranewithout liquid electrolyte.

The terms “alkaline fuel cell,” “fuel cell,” “cell,” used to disclosethe inventions below refer to an alkaline fuel cell including analkaline aqueous electrolyte (AFC) or an OH⁻ ion-conducting polymericmembrane without liquid electrolyte (AMFC). The inventions are notlimited to either type of alkaline fuel cell and may be used with AMFCsand AFCs.

The first air filter or trap 12 of the two-filter combination is achemical CO₂ filter having a high CO₂ absorption capacity and high airthroughput that is designed and configured for thermal rejuvenationwithout requiring disassembly of the filter 12. The filter 12 isdesigned and configured to lower the CO₂ level in an air stream by apredetermined amount, e.g., a reduction by a factor of 10 or from about400 ppm to about 40 ppm in ordinary air, as the air stream passesthrough the filter 12. The first filter 12 is also designed andconfigured to provide a combination of a high capacity of CO₂absorption, e.g., about 5% to 8% by weight, that may be achieved underhigh throughput conditions, e.g., corresponding to air residence timesin the filter 12 of at most about one second. These combined features ofabsorption capacity and dynamic throughput help to lower the level ofCO₂ in the air stream that is ultimately provided to a cathode inlet ofan alkaline fuel cell.

For instance, the first filter or trap 12 may have about 2 kg of activematerial per kW of power generated by the fuel cell and specificationsthat enable a high CO₂ absorption capacity and a high throughput,whereby the filter 12 in a 1 kW cell stack helps to lower the level ofCO₂ in the air stream from about 400 ppm to about 40 ppm or by a factorof 10 during operation of the fuel cell, where the air stream passingthrough the filter 12 has temperatures of up to about 45 degrees C. Thefilter or trap 12 having such specifications may operate for up to about8 hours after which it may become saturated with CO₂ and can besubsequently thermally rejuvenated.

The first filter or trap 12 is constructed of one or more activematerials that enable thermal rejuvenation by removing and releasing theabsorbed CO₂. Such rejuvenation of the first filter 12 is achievedwithout disassembly of the filter 12, and preferably in-line, by passinga stream of warm or hot air, e.g., via a thermal swing absorption (TSA)technique, through the filter or trap 12 to release the absorbed CO₂.

Thus, the first filter or trap 12 may be designed and constructed tomeet the demands of thermal conditions for CO₂ uptake and releasewhereby the filter 12 provides a strong reduction in the level of CO₂ inthe incoming air stream at operation temperatures of the alkaline fuelcell while, at the same time, enables the CO₂-saturated filter 12 torelease absorbed CO₂ at temperatures sufficiently low to avoid excessiveheating energy. Alkaline earth hydroxide materials have been employedfor air CO₂ reduction, but typically require excessively hightemperatures for thermal release of captured CO₂. In addition, the bulkof some active materials, such as oxide/hydroxide granules, aresusceptible to dimensional changes that occur on carbonation and cannotundergo multiple TSA cycles without losses of absorption capacity.

Applicants have identified alternative active materials for constructionof the first thermally regenerative filter or trap 12 including a familyof CO₂ sequestration materials based on polymers with amine functionalgroups¹ that exhibit superior performance for the specific CO₂absorption and reduction applications disclosed herein and that arerequired in achieving CO₂ immunity in alkaline fuel cells. For example,in one configuration of the two-filter assembly 50 according to theinvention, the first filter or trap 12 is constructed of an activematerial, such as a polymer resin with amine functional groups supportedon a porous ceramic substrate, which can provide the required, combinedproperties of CO₂ uptake and desorption at near room temperatures. Theactive material of an amine-functionalized resin and porous ceramicsubstrate have demonstrated CO₂ uptake of about 5% to 8% by weight atnear room temperatures and full CO₂ desorption on exposure to air attemperatures not significantly higher than about 100 degrees C., with aminimum loss of fuel cell performance over multiple TSA cycles. Theseeffects were achieved while also achieving a drop by a required ordesired amount, e.g., a factor of 10, in the CO₂ level in the air streamafter passing through the filter 12, e.g., at residence times of lessthan or not more than one second. Selection of the one or more specifictypes of active materials² of the first air filter or trap 12,therefore, is an important aspect of the solution of achieving CO₂immunity in alkaline fuels cells according to the invention. ¹ Drese, J.H., et al., Advanced Functional Materials, 2008, Vol. 19, pp.3821-3832.² Id.

In addition, as described below in detail with reference to FIG. 4, thethermal regenerative filter or trap 12 may be rejuvenated with thepassage of warm or hot air through the filter 12 to release CO₂ from theCO₂-saturated filter 12. The reductions in the level of CO₂ in the airstream that the regenerative filter 12 accomplishes at the predeterminedamount, e.g., reduction by a factor of 10 or from about 400 ppm to about40 ppm, have been shown to be maintained after multiple cycles ofthermal rejuvenation of the filter 12.

Such active material(s) suitable for achieving the required combinationof properties of the first air filter or trap 12 include, but notlimited to, polymers with amine functional groups and polymers withamine functional groups supported on porous ceramic materials.

The second air filter or trap 14 is a disposable, strongly bonding CO₂chemical filter. For example, in one configuration of the two-filterassembly 50 according to the invention, the second filter or trap 14includes as an active material granules of inorganic hydroxide orhydroxide mixtures that help to effectively lower the CO₂ level by arequired or desired amount, e.g., a reduction by a factor of 10 or fromabout 40 ppm to near 1 ppm, in the exit air stream the strongly bondingCO₂ filter 14 receives from the first filter 12. For instance, when suchfilter 14 is presented with an air stream having a CO₂ level at about10% of ordinary air content, the filter 15 may help to reduce the CO₂level in the air stream down to near 1 ppm. Suitable active filtermaterial(s) of the second filter or trap 14 include materials that arestrong binders of CO₂, which is a property that is required to achievesuch low CO₂ exit levels. Such active filtration material(s) of thesecond filter 14 include, but are not limited to, soda lime, lithiumhydroxide, potassium hydroxide, and sodium hydroxide.

The strongly bonding filter or trap 14 is not capable of rejuvenation atreasonable temperatures and, therefore, requires replacement when theactive material is CO₂ saturated. However, the frequency of replacementof the second filter 14 is relatively low due to the design of thetwo-filter assembly 50 according to the invention, whereby the secondfilter 14 is relatively limited to handling an incoming air stream withonly about 10% of the CO₂ level of ordinary air.

Thus, when the filtration assembly 50 according to the invention asshown in FIG. 1 is incorporated with an alkaline fuel cell, the assembly50 helps to achieve CO₂ immunity within the cell through a series of CO₂filtrations/absorptions that help to significantly reduce the CO₂ levelin the air stream, e.g., from 400 ppm to near 1 ppm where ordinary airis used as the air supply, prior to delivery of the air stream to thecathode inlet. Such significant reduction of the CO₂ level in the airstream supply to the cathode inlet is achieved with minimum maintenanceof the two-filter assembly 50 and minimum energy loss from the fuelcell.

In one configuration of the two-filter assembly 50 according to theinvention, the first CO₂ filter or trap 12 is constructed of an activematerial(s) including, but not limited to, polymer(s) with aminefunctional groups configured to serve as CO₂ trapping sites. Thereaction of the amine(s) with CO₂ and water vapor form bicarbonateaccording to the process:R—NH₂+CO₂+CO₂+H₂0=R—NH₃ ^(+(HCO) ₃ ⁻)  (4)where R may include a carbonaceous polymer backbone.

Further, in another configuration of the two-filter assembly 50according to the invention, the first CO₂ filter or trap 12 isconstructed for use in dry air conditions and includes an activematerial(s) including, but not limited to, polymer(s) with aminefunctional groups configured to serve as CO₂ trapping sites. Thereaction of the amines with CO₂ under dry air conditions form carbamateaccording to the process:2(R—NH₂)+CO₂=(R—NHCOO⁻)(R—NH₃ ⁺)  (5)wherein R may include a carbonaceous polymer backbone.

In addition to the two-filter assembly 50 shown in FIG. 1, cell loadperturbations may be applied to an operating alkaline fuel cell to helpto achieve electrochemical decomposition of any carbonate that maybuildup in the anode portion of the cell and to help to exhaust the CO₂that forms as a result of electrochemical decomposition through theanode exhaust stream. Such an electrochemical CO₂ purging methodaccording to the invention employs a fuel cell load perturbation oflimited duration that passes a maximum current through the fuel cellstack for a relatively short time to help to effectively remove residualcarbonate from the cell, while minimizing the duration of loss of fuelcell power supply to the load that may be incurred when high cellcurrents pass through the fuel cell. The phenomenon of electrochemicalpurge of CO₂ in alkaline membrane fuel cells has been described andserves as a process or technique for restoration of CO₂-free performancein such fuel cells. Applicants have identified that electrochemicalpurging alone cannot be relied upon to achieve CO₂ immunity in analkaline fuel cell because the frequency and the duration of therequired high-current perturbations are prohibitive in terms of theauxiliary power unit that is required to back up the fuel cell and thenet time remaining for cell power supply to the load.

Applicants, however, have identified that use of an electrochemicalpurge approach provides real value in achieving CO₂ immunity in alkalinefuel cells when electrochemical purging is employed in conjunction withCO₂ filtration or absorption, as described above, to reduce the CO₂levels, e.g., from about 400 ppm to about 20 ppm or less, in the airstream entering the cathode portion of the fuel cell. Such filtration orabsorption, as mentioned, is accomplished upstream from an inlet to thecathode using the two CO₂ filter assembly 50 according to the invention,or using the CO₂ system 100 according to the invention as described indetail below. Under lower entry levels of air CO₂, accumulation ofcarbonates within the anode portion of the fuel cell takes relativelylong and, consequently, current perturbations of the fuel cell arerequired relatively infrequently. When the two-filter assembly 50including the thermally regenerative filter 12 and the strongly-bondingCO₂ filter 14 are used upstream to the cathode inlet, theelectrochemical purge method functions as a “polishing” tool that helpsto correct a slow buildup of carbonates in the anode that may resultfrom, for instance, any imperfection in the functions of either filter12 and 14.

The electrochemical purging method according to the invention enableselectrochemical removal of CO₂ from an alkaline fuel cell when theordinary anode process cannot support a demand current due to thereplacement of a large fraction of OH⁻ ions in the anode by carbonateions. Under such conditions, the carbonate ion can replace the OH⁻ ionas a reactant in the anode process according to:1/2 H₂+HCO₃ ⁻=H₂O+CO₂ +e,  (6)thereby “freeing” CO₂ to leave the fuel cell through the anode exhauststream. The process shown by equation (6) is followed by instantaneousfilling of the anionic sites emptied by the electrochemicaldecomposition of carbonate ions with OH⁻ ions migrating into and throughthe anode. The process of anodic carbonate decomposition, therefore,occurs while the anionic current through the thickness dimension of thecell is maintained according to:(R₄N+HCO₃ ⁻)+½H₂+OH⁻=(RN₄ ⁺OH⁻)+CO₂+H₂O+e.  (7)

The key for removal of carbonate from an alkaline fuel cell by suchanodic decomposition, therefore, is temporary electrochemicalperturbation by an application of the maximum current possible to helpto force participation of the carbonate in the anode process. At thesame time, such a temporary load modification, which helps to ensure thedesired process shown in equation (6), involves stack operation atpractically zero power output levels. Consequently, additional power canbe provided for the duration of the perturbation process and can beprovided from an auxiliary power source, e.g., an ultra-capacitor, or abattery. In addition, to help to ensure overall high conversionefficiency, the fraction of operation time used for repetitiveelectrochemical rejuvenation of the fuel stack would not be larger thanseveral percentage points, e.g., from about 1% to about 10%.

Therefore, referring to FIG. 2, another aspect of the invention providesa method 200 of achieving CO₂ immunity in an alkaline fuel cellincluding CO₂ filtration of an air stream to a cathode inlet 32 of thefuel cell employing the two-filter assembly 50 according to theinvention, or the CO₂ filtration system 100 according to the inventiondescribed below, and electrochemical perturbation of the fuel cell foranodic de-carbonation and CO₂ release. The method 200 shown in FIG. 2 isexemplary only and the method 200 may be modified, e.g., by adding,removing, and/or rearranging the stages disclosed below.

At stage 102, the method includes providing an alkaline fuel cell with aseries of CO₂ filters or traps 12, 12A or 12B and 14 that is positionedupstream from a cathode inlet of the fuel cell with at least a firstthermally regenerative chemical CO₂ filter or trap 12, 12A or 12Barranged in a tandem configuration with a second strongly bonding CO₂chemical filter or trap 14. The second strongly bonding filter 14 ispositioned between the cathode inlet 32 and at least one of thethermally regenerative filter 12, 12A or 12B. The first filter 12, 12Aor 12B is designed and constructed to provide a predetermined CO₂absorption capacity, e.g., about 5% to 8% by weight, and a required ordesired throughput capacity, e.g., corresponding to air residence timesin the filter 12, 12A or 12B of at most about one second, to reduce theCO₂ level in the air stream exiting the filter 12, 12A or 12B by apredetermined amount, e.g., reduction by a factor of 10. In oneconfiguration of the filter 12, 12A or 12B according to the invention,active material of the filter 12, 12A or 12B includes one or morepolymers with amine functional groups. The strongly bonding filter 14 isdesigned and constructed to further reduce the CO₂ levels in the airstream it receives from the first filter 12, 12A or 12B before the airstream is supplied to the cathode inlet by a predetermined amount, e.g.,reduction by a factor of 10. In one configuration of the assembly 50according to the invention, active material of the second filter 14includes lime soda, lithium hydroxide, potassium hydroxide or sodiumhydroxide.

At stage 104, filtering an air stream supplied to the fuel cell by theair inlet 30 through the first filter 12, 12A or 12B to help to reducethe CO₂ level in the air stream exiting the first filter 12, 12A or 12Bby the predetermined amount, e.g., from about 400 ppm to about 40 ppm,with a predetermined throughput and residence times of air in the firstfilter 12, 12A or 12B e.g., at most or about one second.

At stage 106, filtering the air stream exiting the first filter 12, 12Aor 12B through the second filter 14 to help to reduce the CO₂ level inthe air stream exiting the second filter 14 and entering the cathodeinlet by the predetermined amount, e.g., from about 40 ppm to near 1ppm.

At stage 108, purging the fuel cell electrochemically for CO₂ removal atthe fuel cell anode through anodic decomposition by applying a maximumcurrent to the fuel cell suitable to help to force participation ofaccumulating carbonate ions in the fuel cell anode as a reactant in theanode process (shown by equation (6)), thereby freeing CO₂ for removalfrom the fuel cell through the anode exhaust stream. The magnitude ofthe current is sufficiently high and just short of any magnitude thatwould cause an onset of fuel cell reversal in the stack. Such purgingmay be applied to the fuel cell temporarily and periodically.

At stage 110, maintaining the application of the purging current for apredetermined duration, e.g., of about 1 second to about 30 seconds,such that, a substantial portion of carbonate ions replaces asubstantial portion of OH⁻ ions as a reactant in the anode process andare thereby consumed electrochemically with CO₂ being released as aby-product and released from the fuel cell through the anode exhauststream.

At stage 112, providing optionally during electrochemical purgingstages, when required, supplemental power to accommodate the consequenttemporary load modifications and reduced power output levels of theoperating fuel cell stack. Such supplemental power may be provided by anauxiliary power source, e.g., an ultra-capacitor, or a battery.

Referring to FIG. 3, a chart 52 illustrates a decrease in voltage of analkaline fuel cell operating at a constant current to the load where anair supply to the cell cathode provided with “ultra zero” levels of anycomponents other than oxygen and nitrogen, or is relatively CO₂ free, isswitched to an ambient air supply. The chart also illustratesmaintenance of voltage of the fuel cell using an ambient air supply tothe cell cathode and the two-filter assembly 50 described above, or thefiltration system 100 described below, to reduce the level of air CO₂.Filtration or active capture of CO₂ with the assembly 50 or the system100 may be used in conjunction with the electrochemical perturbationmethod 200 described above for CO₂ removal and release at the fuel cellanode through anodic decomposition.

Referring to FIG. 4, in another aspect, the invention provides a CO₂filtration system 100 for an alkaline fuel cell 20 including a firstthermally regenerative filter or trap 12A and a second thermallyregenerative filter or trap 12B, each filter or trap 12A and 12B havingthe same properties and specifications as the thermally regenerativefilter or trap 12 described above with reference to FIG. 1. The firstand second thermally regenerative filters 12A and 12B are positioneddownstream from an air inlet 30 and upstream from an inlet 32 of thefuel cell cathode. The first and the second thermally regenerativefilters 12A and 12B are disposed in a parallel orientation to oneanother. In addition, each filter 12A and 12B is positioned upstreamfrom and in a tandem configuration with the strongly bonding CO₂ filteror trap 14, which has the same properties and specifications asdescribed above with reference to FIG. 1. The strongly bonding filter 14is positioned upstream from the cathode inlet 32 and receives thefiltered exit air stream from either the first filter 12A or the secondfilter 12A, depending the mode of operation of each filter 12A and 12B,as described below. The air pump 16 is disposed between the thermallyregenerative filters 12A and 12B and the strongly bonding filter 14 toinduce flow of an air stream through the filters 12A, 12B and 14 andinto the cathode inlet. The system 100 is constructed and arranged toprovide a thermal rejuvenation scheme that allows one of the filters 12Aor 12B to actively filter CO₂ while the other filter 12A or 12Bundergoes thermal rejuvenation, if needed.

The first and second thermally regenerative filters or traps 12A and 12Bare configured and designed to enable thermal rejuvenation, e.g., via athermal swing absorption (TSA) technique, by passing a warm or hot airstream through the filter 12A and 12B to release absorbed CO₂. Thethermally regenerative filters 12A and 12B are operatively connected toand arranged with a subsystem of air flow lines 22 and valves V₁, V₂,V₃, and V₄, e.g., two-way and/or three-way valves, as shown in FIG. 4.The subsystem is configured and arranged to deliver from the air inlet30 an inlet air stream to each filter 12A and 12B, and to selectivelydeliver the inlet air stream to either filter 12A or 12B depending onwhether filter 12A or 12B is actively trapping CO₂ from the air stream.In addition, the subsystem is also configured and arranged to deliver arejuvenation air stream for thermal regeneration to each filter 12A and12B, and to selectively deliver the rejuvenation air stream to eitherfilter 12A or 12B depending on whether filter 12A or 12B is designatedfor and/or undergoing thermal rejuvenation. The subsystem delivers theappropriate air stream depending on the mode of operation of the filters12A and 12B, delivering the inlet air stream to filter 12A or 12B whenactive for filtering the inlet air stream to reduce the level of CO₂ anddelivering the rejuvenation stream to filter 12A or 12B when undergoingthermal rejuvenation.

For instance, the subsystem can employ one or more of the air flow lines22 and one or more of the valves V₁, V₂, V₃, and V₄ to deliver the inletair stream to filter 12A that is actively trapping CO₂, and can deliver,e.g., simultaneously, the rejuvenation air stream to filter 12B that isundergoing thermal regeneration, or vice versa. The first and secondfilters 12A and 12B and certain of the air flow lines 22 and valves V₁,V₂, V₃, and V₄ can thereby help to enable one of the filters 12A or 12Bto reduce the level of CO₂ in the inlet air stream, while enabling theother filter 12A or 12B to undergo thermal rejuvenation by passing awarm or hot air rejuvenation stream through the filter 12A or 12B. Thesystem 100 according to the invention may operate to ensure that atleast one of the thermally regenerative filters 12A or 12B is alwaysactively trapping CO₂ to reduce the level of CO₂ in the air stream thatwill subsequently be supplied to the strongly bonding filter or trap 14.

In one configuration of the subsystem according to the invention,certain airflow lines 22 and valves V₁, V₂, V₃, and V₄ are configuredand arranged to redirect the warm or hot cathode exhaust air stream tothe first and second filters 12A and 12B, such that, the cathode exhauststream serves as the rejuvenation stream as it passes through filter 12Aor 12B, depending on whether filter 12A or filter 12B is designated forand/or undergoing thermal regeneration. The system 100 according to theinvention thereby implements in-line thermal rejuvenation of the firstand second filters 12A and 12B without requiring disassembly of thefilters 12A and 12B. Such in-line rejuvenation can be performed duringoperation of the fuel cell 20, such that, at least one of the first andsecond filters 12A and 12B, either filter 12A or 12B, is dedicated toreceiving and filtering the inlet air stream.

In one configuration of the system 100 according to the invention, theredirected cathode exhaust stream serving as the rejuvenation air streammay include additional or supplemental heating provided by an in-lineheater 24, e.g., an electric or catalytic heater, operatively connectedwith one or more of the air flow lines 22 and/or one or more of thevalves V₁, V₂, V₃, and V₄ of the subsystem, to help to increasetemperatures of the rejuvenation air stream to the required or desiredrejuvenation temperatures. Such an in-line heater 24 may use somehydrogen fuel of the fuel cell 20 for its operation.

Thermal release of CO₂ is achieved by passing the rejuvenation airstream through filter 12A or 12B at temperatures within a range of fromabout 80 degrees C. to about 120 degrees C., and preferably from about100 degrees C. to about 105 degrees C. In addition, the configurationand operation temperatures of the filters 12A and 12B ensure that thetime required for the filter 12A and 12B to recover CO₂ absorbingcapacity is less than the CO₂ saturation time under equal airflow ratesduring the adsorption and desorption half cycles. Subsequent to passingthrough the filter 12A or 12B undergoing thermal regeneration, therejuvenation air stream may be released from the subsystem via the CO₂regeneration air stream outlet 34

One of more of the air flow lines 22 and one or more of the valves V₁,V₂, V₃, and V₄, are also configured and arranged to deliver to thestrongly bonding filter 14 the exit air stream from either the first orsecond filter 12A or 12B for further CO₂ absorption by the filter 14 asthe air stream passes through the filter 14. One or more of the air flowlines 22 and one or more of the valves V₁, V₂, V₃, and V₄ are configuredand arranged to deliver the exit air stream from the filter 14 to thecathode inlet 32. At least one air flow line 22 receives an inlet airstream from the air inlet 30 to direct flow of the air stream to thefirst or second filter 12A or 12B, depending on which filter 12A or 12Bis engaging in filtering the air stream. An air contaminants filter 17may be operatively coupled to this air flow line 22 to help to removeany contaminants present in the inlet air stream.

As described, the system 100 and, in particular, the subsystem of airflow lines 22 and valves V₁, V₂, V₃, and V₄, enables operation of thepair of thermally regenerative filters 12A and 12B in different modeswhereby one mode includes filter 12A or 12B actively trapping CO₂ and asecond mode includes filter 12A or 12B undergoing thermal regeneration,e.g., via redirection of the exhaust cathode stream through such filter12A and 12B. For instance, filter 12B may undergo thermal rejuvenationwhile, at the same time, filter 12A is actively trapping CO₂ in the airstream. Switching the modes of operation of each filter 12A and 12B fromactively trapping CO₂ to thermal rejuvenation and then back to activelytrapping CO₂ may be accomplished after a preset period of time ofoperation of the fuel cell 20 at some given fuel cell power output.After expiration of the preset period of time of operation, the airstreams within the fuel cell 20 may be redirected by one or more of thevalves V₁, V₂, V₃, and V₄ and one or more of the air flow lines 22, suchthat, the exhaust cathode stream may be re-directed to filter 12A or 12Bfor thermal rejuvenation and the inlet air stream may be directed tofilter 12A or 12B for active trapping of CO₂ in the air stream.

The inventions disclosed above with reference to FIGS. 1 and 2 and FIG.4 provide flexibility in addressing any particular application forreduction of the CO₂ level in the air stream provided to the cathodeinlet 32 and for ultimately achieving CO₂ immunity in an alkaline fuelcell 20. In particular, the two-filter assembly 50 or the system 100 maybe used alone or in conjunction with the method 200 of electrochemicalperturbation to reduce the CO₂ level. In addition, reduction of the CO₂level in the air stream may also be accomplished using only one type ofthe two types of filters 12 and 14 of the two-filter assembly 50, withor without use of the electrochemical perturbation method 200.Similarly, reduction of the CO₂ level in the air stream may also beaccomplished using only one of the pair of thermally regenerativefilters 12A and 12B of the system 100, with or without use of theelectrochemical perturbation method 200. The options would depend uponthe particular application, the specifications of the CO₂ filters, andthe efficiency of the electrochemical perturbation method 200 in analkaline fuel cell; and, would depend on a given membrane and electrodematerials and their specifications. Some of such options are summarizedbelow:

(1) Using one of the pair of thermally-regenerative filters 12A or 12Bof the system 100 for active CO₂ absorption while the other filter 12Aor 12B is undergoing thermal rejuvenation in order to maintain CO₂absorption and thereby reduction of the CO₂ level in the air stream atall times during operation of the alkaline fuel cell.

(2) Using only one of the thermally regenerative filters 12, 12A or 12B,where the air stream passes only through the strongly bonding CO₂ filter14, while the thermally regenerative filter 12, 12A or 12B undergoesthermal rejuvenation.

(3) Using only the strongly bonding CO₂ filter 14 upstream from thecathode inlet 32 in conjunction with the method 200 of electrochemicalperturbation, when required. This option is desirable where thefrequency of manual replacement of the filters 14 is dictated bysuitable dimensions of the filter 14 and is operationally acceptable.

(4) Any option involving thermal regeneration of one of the filters 12,12A or 12B where at least some of the thermal energy used for thethermal rejuvenation is derived from the re-direction of the cathodeexhaust stream through the filter 12, 12A or 12B.

One of ordinary skill in the art can appreciate that the inventionsdisclosed are not limited to the options described above and theinventions envision other possible combinations of these CO₂ absorptionand release capabilities that the two-filter assembly 50 or the CO₂filtration system 100 can provide to a given alkaline fuel cell and itsstack subsystems, depending on the given operating conditions andspecifications of the filters or traps 12, 12A, 12B and 14 and given thefuel cell, to help to achieve CO₂ immunity within the fuel cell.

Referring to FIG. 5, in another aspect, the invention provides a method300 of determining the minimum overall dimensions of a design of the CO₂filtration assembly 50 or the system 100 according to the invention tohelp to ensure the reduction of CO₂ levels in an air stream to thecathode inlet 32 to ultra-low CO₂ levels. The method 300 is exemplaryonly and may be modified, e.g., by adding, removing, and/or rearrangingstages.

At stage 302, determining a maximum CO₂ level in an air stream to thecathode inlet 32 of the alkaline fuel cell 20 that would cause a loss offuel cell power at a maximum predetermined percentage.

At stage 304, determining a volume of the strongly bonding filter ortrap 14 that is required for the filter or trap 14 to lower CO₂ levelsin the air stream exiting the thermally regenerative filter or trap 12,12A and 12B.

At stage 306, determining a volume of the strongly bonding filter ortrap 14 that is required to contain and/or to maintain the filter's 14active material to trap CO₂, e.g., at about 30 to 40 ppm, over theshortest period of time acceptable for replacement of the filter 14.

At stage 306, determining the weight of the thermally-rejuvenated activematerial of each thermally regenerative filter or trap 12, 12A and 12Brequired to lower CO₂ levels in the air stream to the cathode inlet,e.g., from about 400 ppm to about 30 to 40 ppm or by a factor of 10, ata given air flow rate to the fuel cell over duration of a typical “On”period of a given duty cycle, e.g., 8 hours, and preferably to help toaccomplish thermal rejuvenation during the fuel cell “Off” period.

Having thus described at least one illustrative aspect of the invention,various alterations, modifications and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements are intended to be within the scope and spirit of theinventions disclosed above. Accordingly, the foregoing description is byway of example only and is not intended as limiting. The invention'slimit is defined only in the following claims and the equivalentsthereto.

What is claimed is:
 1. An alkaline fuel cell comprising: an air inletoperatively coupled with a three-trap air filter assembly; thethree-trap air filter assembly including a first thermally regenerativechemical CO2 trap and a second thermally regenerative chemical CO2 trap;and a third disposable CO2 chemical trap; each of the first and secondthermally regenerative chemical traps being arranged parallel to oneanother upstream from and in a tandem configuration with the thirddisposable CO2 chemical trap, the first and second thermallyregenerative chemical traps and the third disposable chemical CO2 trapall being disposed downstream from the inlet, the third disposablechemical CO2 trap disposed upstream from an inlet to the cathode of thealkaline fuel cell; each of the first and second thermally regenerativechemical traps being configured to reduce the level of CO2 in an airstream passing through a thermally regenerative chemical trap by a firstpredetermined amount, and the third disposable chemical CO2 trap beingconfigured to reduce the level of CO2 in an air stream passing throughthe third disposable chemical CO2 trap by a second predetermined amount;and each of the first and second thermally regenerative chemical trapsbeing switchable to different modes to switch one thermally regenerativechemical trap to actively trap CO2 in an air stream passing through theone thermally regenerative chemical trap while the other thermallyregenerative chemical trap undergoes thermal regeneration, wherein thealkaline fuel cell includes an alkaline aqueous electrolyte or an OH-ionconducting polymeric membrane without liquid electrolyte.
 2. Thealkaline fuel cell of claim 1, wherein each of the first and secondthermally regenerative chemical CO2 traps is configured to switch from aCO2 trapping mode to a thermal rejuvenation mode and back to a CO2trapping mode after a preset period of operation of the fuel cell at agiven fuel cell power output.
 3. The alkaline fuel cell of claim 2,wherein each of the first and second thermally regenerative chemical CO2traps is configured to switch from a CO2 trapping mode to a thermalrejuvenation mode by redirection of inlet air and cathode exhaust airstreams, wherein cathode exhaust air is redirected to each thermallyregenerative chemical CO2 trap for thermal rejuvenation.
 4. The alkalinefuel cell of claim 1, wherein the predetermined amount includes a factorof 10, and the second predetermined amount includes a reduction by afactor of
 10. 5. The alkaline fuel cell of claim 1, wherein the firstand the second thermally regenerative chemical CO2 traps are configuredfor thermal regeneration by passing a rejuvenating air stream throughthe thermally regenerative chemical CO2 traps wherein the rejuvenatingair stream includes cathode exhaust air supplied to the thermallyregenerative chemical CO2 traps with or without additional heating. 6.The alkaline fuel cell of claim 5, wherein any additional heatingincludes heating provided by an electric heater or a catalytic heateroperatively coupled with the fuel cell.
 7. The alkaline fuel cell ofclaim 1, wherein the disposable chemical CO2 trap includes an activematerial of inorganic hydroxide or hydroxide mixtures, and wherein theactive material includes one or more of: soda lime, lithium hydroxide,potassium hydroxide or sodium hydroxide.